I know a long coil of tubing in a cooler of ice can keep beer cold for drinking. If you just use blocks of ice you can dip the cooled water out and add more as you need it. Many of the installers using self-levelers in the hot country will use 10 - 15 blockss of ice to chill their water. Try to keep the water temp below 70F. It doesn't have to be as cold, as does beer.
Rayt